1. The Field of the Invention
This invention relates generally to the field of physical vapor deposition for creating thin films on a substrate. In particular, embodiments of the present invention relate to a dynamically adaptable mask for use in a physical vapor deposition process.
2. The Relevant Technology
Physical vapor deposition (PVD) is a process for depositing specific “target” materials onto a substrate. The process generally involves liquefying or vaporizing a portion of the target material such that it is eventually deposited onto the substrate. Generally, if chemical reactions are taking place at the substrate by design, then the process is usually called chemical vapor deposition (CVD). If the material is collecting on the surface and not much more than nucleation is going on, as in the case of most metals deposition, then the process is called PVD. The PVD process is commonly used in the field of optics to deposit thin films of specific materials onto substrates. These thin films generally have optical properties such as reflection, transmission, absorption, or polarization. The process can also be used to deposit specific patterns of materials onto a substrate for the purpose of creating optical or electrical effects. For example, a specific pattern can be deposited on a geometrical optical element for only reflecting light that is contained within the specific pattern deposited by the PVD process.
There are many different techniques for providing the necessary bond-breaking energy to the molecules within the target material to initiate the necessary state change. The most common PVD techniques are flash evaporation, electron beam (E-beam) evaporation, ion-plating, and pulsed laser deposition (PLD). Flash evaporation involves heating up a container device to a temperature above the melting point of the target material. The target material is then slowly inserted into the container in a delayed manner such that all of the previously inserted target material has evaporated before more is inserted. Flash evaporation generally causes some undesirable splattering of the target material. The most common form of flash evaporation is a simple coiled filament, not unlike what is found in light bulbs and vacuum tubes. For flash evaporation, the filament current is pulsed way beyond that required to melt the material and it explosively evaporates. Sometimes the material is coated on a much more refractory filament material, and sometimes the material is heated to just below the melting point such that enough material sublimes from the hot filament to do a slow deposition.
E-beam evaporation requires the target material to be placed in a water cooled container. A specific portion of the target material is then heated by a powerful high voltage electron beam. The electron beam has a high flux causing the specific portion of the target material to be sublimated while the remaining target material is cooled by the water cooled container. Sublimation refers to heating up a solid material so as to cause it to skip the liquid phase and instantly become vaporized. Unfortunately, this technique requires large amounts of energy to be used in powering the electron beam source. While some materials do sublime, most melt and evaporate since there is usually a significant temperature difference between the melting point and the boiling point. Also, to keep from forming pockets and distorting the evaporation plume profile, as well as reduce the particulates, the beam is scanned across the surface of the target so that the target erodes as evenly as possible.
In ion plating, the target material is evaporated into a plasma state by an energy source, such as a high voltage electron beam. The evaporated material alternates between a charged state as positive ions and an uncharged state as neutral atoms. When the evaporated material is in the positive ionic state, the plasma affects the transport of the molecules so that it appears to the substrate that the plasma is a diffuse source of the evaporated target material rather than the evaporation source itself. Long-term use of this process poses significant obstacles in preventing the energy source from being corroded by the plasma. Like E-beam evaporation, this process also requires a tremendous amount of energy to power the energy source required to maintain the necessary heat to keep the target material in a plasma state.
Pulsed laser deposition (PLD) is emerging as one of the most popular forms of PVD for many reasons. It has been determined that PLD can be used to deposit complex combinations of elements or other substances in their original composition onto a substrate. This is extremely important because different elements each have unique melting points, meaning that it is difficult to proportionally vaporize all of the materials within a composition. This is achieved by using non-equilibrium surface heating of the multi-element composition target material. Other PVD processes are not capable of precisely controlling the uniformity of heat from the source to the target material. In addition, PLD is generally able to precisely deposit portions of target material with less contamination problems than other PVD processes. This precision is important for depositing specific patterns of target material on a substrate.
Contamination is a problem when external compositions are deposited onto the substrate in addition to the target material. For example, if a PVD process utilizes a ceramic container to contain the liquid target material before it is deposited, a foreign substance may vaporize within the ceramic material and be added to the target material.
Pulsed laser deposition uses a laser beam to provide the necessary bond-breaking energy to the target material. A portion of the target struck by the laser is evaporated. The wavelength of the laser affects the depth at which the laser is absorbed and the amount of target material which is consequently heated. For example, ultraviolet radiation is typically absorbed to a shallow depth of about 1,000 Å. The laser is pulsed so as to not continue boring a hole through the target material. The key to successful ablation is power density. Thus, a lot of power is used generally for a short period of time so that one does not bore a hole through the target material. The high power can be achieved by focusing or other methods but it is the energy per unit time in a given area that determines the effectiveness of PLD—Joules/sec/cc. Thus, the ablation increases when the energy is increased, the time, pulse duration, is shortened, or the volume is made smaller. The surface of the target material is rapidly heated up to thousands of degrees Celsius, while the bottom of the target material remains at approximately room temperature. This non-uniform heating of the target material causes a flash of evaporants to be released. The evaporants of target material form a “plume” and are eventually deposited onto the substrate. The resulting deposit on the substrate will be a thin film with exactly the same composition as the target material.
It is common to use masks with physical vapor deposition processes, which enable material to be selectively deposited over the regions of a substrate that are exposed through the mask. All PVD processes utilize some form of source for generating the necessary bond-breaking energy to deposit the target material onto a substrate. Unfortunately, almost all sources exhibit wear and drift characteristics over time. The target also wears and changes over time. In fact, it is quite common to have more than one set of masks that are changed out according to the wear on the target. Currently, this requires interrupting the run and opening the chamber. Therefore, the characteristics of a mask that are used in conjunction with a changing source should also be capable of changing. Currently, masks are simply mechanically modified in an essentially experimental manner in an effort to compensate for these changes. Unfortunately, this process generally results in non-uniform depositions due to the crude adjustments made to the mask.